As of 2009 GaN LED's or light emitting diodes can produce blue wavelength light at >55% wall-plug efficiency. 55-65% wall plug efficiency for the die sources refers to the production of 550-650 mW of blue light at a wavelength between 405-489 nm for every watt of electrical power. The problem with blue light emission is that the photopic efficacy spectrum requires a large amount of green light to produce luminous flux and a white spectra requires a continuous power from 380-780 nm where in light in the green-red spectrum is of highest importance in the warm white approximating the spectral distribution of a 2700-3000K planckian radiator. Down converting the blue light to yellow and red produces stokes shift quantum loss which is unavoidable. Phosphors available in the yellow, orange, and red wavelengths also incurs internal quantum efficiencies of typically 70-90%. At an operating temperature of 100 deg C. the phosphors down-convert light less efficiently through a non-radiative process known as thermal quenching.
For example the typical wavelength conversion efficiency of a red phosphor at a temperature of 115 deg C. is 78%. When combined with the internal quantum efficiency of the phosphor of at best 90% and the stokes shift conversion loss (78-82%) the typical efficiency of a blue/red conversion is approximately (0.82)*(0.9)*(0.78)=0.574 or 57%. The thermal quenching of the phosphor is most problematic when the phosphor layer is directly deposited to the die structure through an electrophoretic deposition process, or through the heat generated at the chip which reduces the wavelength conversion efficiency of a luminescent ceramic chip placed directly on the blue die emitter itself, or when the phosphor loaded silicone or epoxy surrounds the blue or UV LED emitter in the same cavity.
Several solutions have been proposed to reduce the problem of thermal quenching and phosphor scattering, including the placement of a thin layer of phosphor loaded silicone remotely from the LED light source but contained in the same LED cavity, or to introduce a dichroic mirror at the exit of a confocal-parabolic concentrator where the blue light excites the phosphor, and then the green-red light is forward reflected giving the opportunity for the back-reflected blue light to recycle and then re-excite the phosphor.
The problem with the remote phosphor methods previously proposed by others is that the layer produces a larger source thereby reducing luminance, and still backscatters light into the high index die emitter due to the unavailability of a suitable high index encapsulant to match the die structure and remotely located phosphor composite. One of the shortcomings of the dichroic mirror approach is that the reflectance and transmission efficiency is highly angle dependent, costly to implement, and not manufacturable at the volumes required for an incandescent light bulb replacement. The problem with the luminescent ceramic chip placed directly on the die emitter is that the wavelength conversion will degrade with temperature as the chip heats up. All of these prior methods have the added shortcoming of only producing light in a 180 deg hemisphere which only fills half of the solid angle of distribution produced by the household vertical filament incandescent lamp.